The present invention relates to a feed structure for an antenna. Embodiments of the invention find particular application in flexible feed structures for radio antennas, such as those which can be incorporated into clothing.
Wearable antennas have been developed for use in variety of communications applications. The construction of an antenna using flexible materials has been investigated and can give a relatively discreet result which does not hinder the wearer's movements.
There are several challenges in developing a wearable antenna which can for example be incorporated into clothing. One of these is the feed for delivering communications signals to/from the antenna, these signals normally being at radio frequencies. The feed itself needs to deliver sufficient power while being relatively undetectable and also robust, for instance to withstand normal movement and handling of the clothing, and washing.
A dipole antenna is a form of antenna known for use in a wearable construction but, in practice, it requires a balanced feed in order to prevent the feed itself from radiating as well as the antenna. If the feed radiates, it reduces the efficiency of the antenna, can distort the radiation/reception pattern and can interfere with other equipment. The output of a radio for use with a wearable communications antenna is unbalanced. It is known to use a transmission line plus a balun to convert the radio output to a balanced antenna feed.
Other constraints with regard to an antenna feed suitable for wearable antennas are that it should be compatible with broadband operation and deliver an adequate signal power for use in the field, for example 5 Watts or more.